The present invention relates to an ultrasonic probe having a plurality of ultrasonic transducer elements arranged in a row.
A conventional ultrasonic probe of this type is shown in FIGS. 1 and 2. Ultrasonic probe 1 in FIGS. 1 and 2 has a plurality of ultrasonic transducer elements 2-l to 2-n arranged in a row. Signal electrodes 3-l to 3-n are provided at one side of transducer elements 2-l to 2-n, respectively. Earth electrode 4 is provided at the other side of a plurality of transducer elements 2. Backing member 5 for absorbing an unnecessary ultrasonic wave is provided adjacent to signal electrodes 3-l to 3-n. A plurality of signal conductive members 6-l to 6-n for leading an electrical signal are connected to signal electrodes 3-l to 3-n, respectively. Signal conductive members 6-l to 6-n extend parallel to each other on the upper surface of backing member 5. Plate-like earth conductive member 7 for earthing the transducer elements is connected to earth electrode 4. Earth conductive member 7 is arranged on the lower surface of backing member 5. Matching layer 8 and acoustic lens 9 are provided adjacent to earth electrode 4.
Therefore, driving signals are sequentially supplied from a transmitter/receiver (not shown) to signal electrodes 3-l to 3-n through signal conductive members 6-l to 6-n, at each delay time. As a result, transducer elements 2-l to 2-n sequentially emit ultrasonic waves toward acoustic lens 9 at predetermined times. These ultrasonic waves are synthesized to define an ultrasonic beam. This ultrasonic beam is deflected and scans a human body. The ultrasonic beam (echo) reflected by an interior of the human body is detected by the transducer elements, and a tomographic image of the human body is displayed on a cathode-ray tube (not shown).
A flow rate of blood flowing through a heart or a blood vessel is sometimes measured by a so-called continuous wave Doppler mode (CWD mode). That is, a plurality of transducer elements, a plurality of earth electrodes, and a plurality of signal conductive members are divided into first group for generating ultrasonic waves and second group for receiving ultrasonic waves (echoes). When driving signals are supplied to signal electrodes of the first group, transducer elements of the first group generate ultrasonic waves continuously. These ultrasonic waves are reflected and detected by transducer elements of the second group. In this case, because of a Doppler effect, a frequency of the reflected ultrasonic wave differs from that of the generated ultrasonic wave. This difference between the two frequencies is proportional to a flow rate of the blood. As a result, this frequency difference is calculated, and the flow rate of the blood is measured and displayed on a cathode-ray tube (not shown).
As shown in FIG. 3, a pair of parallel conductive wires A and B extend perpendicularly to the sheet of the drawing Conductive wires A and B are separated from each other by distance d and have height h from the earth.
Assume that current I is supplied to conductive wires A and B in the same direction. In this case, mutual inductance M represented by the following equation (1) is emerged between conductive wires A and B: EQU M=(.mu./4.pi.)log.sub.e[{ d.sup.2 +(2h).sup.2}/ d.sup.2 ][H/m](1)
where M is a mutual inductance per unit length between wires A and B and .mu. is a permeability of a medium.
It is known that as mutual inductance M is increased, an amount of crosstalk or coupling generated between conductive wires A and B is increased. This crosstalk or coupling is a phenomenon in which an electrical signal transmitting through conductive wire A is emerged in conductive wire B and that an electrical signal transmitting through conductive wire B is emerged in conductive wire A. As is apparent from equation (1), as distance d between conductive wires A and B is reduced or height h between the conductive wires and the earth is increased mutual inductance M is increased, and the crosstalk is increased.
In the conventional ultrasonic probe shown in FIG. 1, assume that a distance between the signal conductive members is d and a height between the signal conductive members and the earth conductive member is h.
In the conventional ultrasonic probe, in order to improve directivity of an ultrasonic wave, the transducer elements are arranged close to each other. For this reason, distance d between the signal conductive members is relatively small. Therefore, the crosstalk occurs frequently. In addition, the signal or earth conductive member is arranged on the upper or lower surface of the backing member. For this reason, height h between the signal and earth conductive members is relatively large. Therefore, the crosstalk occurs frequently. That is, since the crosstalk occurs frequently, the ultrasonic wave is unnecessarily generated, and the tomographic image formed by a detected ultrasonic wave sometimes causes artifact. In the CWD mode, crosstalk is sometimes generated between the first and second group signal conductive members. For this reason, the flow rate of the blood is not sometimes accurately measured.
Therefore, a demand has arisen for reducing the crosstalk. However, since the transducer elements are arranged very close to each other, it is very difficult to increase distance d between the signal conductive members. For this reason, a demand has arisen for reducing height h between the signal conductive members and the earth, thereby reducing the crosstalk.